Many valuable non-ferrous metals occur as ores or minerals in conjunction with iron in various forms. Examples are nickel, often present as a sulphide such as pentlandite or in an oxidised form in laterites; copper, often present as sulphides such as chalcopyrite, or bornite or in an oxidised form as chrysocolla; and zinc, often present as a sulphide in association with pyrites or in an oxidised form as willemite. These ores are treated directly by pyrometallurgical or hydrometallurgical methods or they are upgraded, especially by flotation in the case of sulphides, to form concentrates. The concentrates are then treated to recover the valuable non-ferrous metals. The ores, minerals or concentrates, particularly if they consist primarily of sulphides, are generally roasted to form an oxide product, hereinafter termed calcine, which is then treated hydrometallurgically to recover the valuable non-ferrous metals.
In many cases the hydrometallurgical treatment involves a leach of the ore, mineral, concentrate, or calcine, to dissolve the valuable metal or metals. Various types of leaching techniques are known to those skilled in the art. They may involve acidic, neutral, or alkaline solutions, containing suitable oxidizing or reducing agents if required.
During leaching in acid solutions it is common for iron to be dissolved simultaneously with the valuable non-ferrous metal or metals, and to report into the leaching solution as ferric iron. Sometimes this ferric iron must be removed from the solution before the valuable non-ferrous metal or metals can be recovered. In some other cases the ferric iron may be removed after the valuable non-ferrous metal has been recovered and before the solutions are recycled for re-use.
The electrolytic zinc process can be cited as an example to demonstrate the type of procedures involved. Typically, zinc sulphide ores are treated by flotation to form a zinc sulphide concentrate which in addition to zinc sulphide, normally contains some iron and frequently small amounts of other valuable non-ferrous metals such as lead, cadmium, copper, silver and gold. When the zinc sulphide concentrate is subjected to an oxidising roast to form a zinc oxide calcine there is a reaction between some of the iron and the zinc in the concentrate to form a zinc ferrite of the type ZnFe.sub.2 O.sub.4. When the calcine is leached in dilute acid, the zinc oxide dissolves as zinc sulphate but leaves the ferrite in the solid residue. This represents a loss of zinc, and in the past this ferrite residue was either stockpiled or treated by pyrometallurgical methods. When the ferrite residue is leached with acid at elevated temperatures both the zinc and the iron are solubilised, and the ferric iron must be removed from the solution before the zinc can be recovered by electrolysis. During the leaching processes other minor valuable metals, such as copper and cadmium, are also dissolved.
In recent years several techniques have been suggested for separating the iron from the leach solution containing ferric iron and the valuable metal or metals. The most widely accepted is the well known Jarosite Process (Australian Pat. No. 401724) in which the ferric iron is precipitated as a complex basic iron sulphate of the form, MFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 where M is H.sub.3 O.sup.+, NH.sub.4.sup.+, Na.sup.+ or K.sup.+. The Jarosite Process has been well established in the electroyltic zinc industry. Iron is also separated from solution as a jarosite in the cobalt industry (see for example, Aird, J., Celmar, R. S., and May, A. V., "New Cobalt Production from R.C.M.'s Chambishi Roast - leach - electrowin Process", Mining Magazine, October 1980, pages 320-326). Precipitation of ferric iron as jarosite has also been suggested for use in the recovery of nickel from laterites (see for example Australian Pat. No. 517492). The ferric iron may also be recovered from solution as compounds of undisclosed form, but probably a compound similar to ferric oxide or goethite (see Australian Pat. No. 424095 and South African patent application No. 75/2737). It may be precipitated as hematite (see for example Mealey, M., "Hydrometallurgy Plays a Big Role in Japan's New Zinc Smelter" in Engineering and Mining Journal, January 1973, pages 82-84). The ferric iron may even be removed from the solution by solvent extraction (see Australian Pat. No. 487596).
In all the examples cited above, the leach solution treated for removal of ferric iron contains some acid in addition to the ferric iron and the valuable non-ferrous metal or metals. The amount of acid is normally adjusted to a concentration which depends upon the methods subsequently used to precipitate the iron.
In the Jarosite process, the acid content of the solution leaving the leaching step and entering the jarosite precipitation must be at least 20 grams of H.sub.2 SO.sub.4 per liter according to the paper by Wood, J. T., entitled "Treatment of Electrolytic Zinc Plant Residues by the Jarosite Process" Australian Mining, January 1973, pages 23-27, and this acid must be neutralized during the jarosite precipitation step of the process. Furthermore, jarosite is precipitated by a reaction of the type: EQU M.sub.2 SO.sub.4 +3Fe.sub.2 (SO.sub.4)+12H.sub.2 O=2MFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 +6H.sub.2 SO.sub.4.
where M is one or more of H.sub.3 O.sup.+, NH.sub.4.sup.30 , Na.sup.+ and K.sup.+. The sulphuric acid liberated must also be neutralized to maximise the precipitation of jarosite. It is clear that to minimise the amount of neutralizing agent required, the acidity of the solution entering the jarosite precipitation step must be as low as possible. Traditionally it has not been possible to reduce the acidity below 30 g/1 because of the formation of jarosite which ultimately reports in the lead - silver residue produced as a by-product, and this is a particularly undesirable feature.
In those processes where a neutralizing agent is added during the ferric iron precipitation stage, such as in the conventional Jarosite Process or where ferric iron is precipitated as ferric oxide, geothite, or related compounds the neutralizing agent is frequently the ore or mineral treated, or the calcine containing the oxidised valuable non-ferrous metal or metals. The residues from these neutralizing agents also contain valuable non-ferrous metals and these are then discarded with the iron waste. This represents a substantial loss of valuable raw materials, and a process which is capable of significantly reducing the amount of neutralizing agent required by reducing the acidity of the feed solution would enable great savings to be made. Where a different neutralizing agent is used, such as limestone, lime or ammonia, a reduction in the amount of neutralizing agent required would represent a considerable saving in costs.
When the iron is precipitated as ferric oxide, goethite, or related compounds by reactions of the type: EQU Fe.sub.2 (SO.sub.4).sub.3 +(x+3)H.sub.2 O=Fe.sub.2 O.sub.3.xH.sub.2 O+3H.sub.2 SO.sub.4 and EQU Fe.sub.2 (SO.sub.4).sub.3 +4H.sub.2 O=2FeOOH+3H.sub.2 SO.sub.4
where x is a number .gtoreq.0, acid is again liberated during the precipitation reaction and, as in the Jarosite Process, this acid must be neutralized together with the acid present in the initial solution. Where these techniques are used to precipitate ferric iron, the solutions contain much lower levels of ammonium, sodium or potassium ions, and the risk of premature precipitation of iron is not as great. As a result, the acidity of these solutions can be lower than that of solutions in the Jarosite Process. The examples in Australian Pat. No. 424095 show that an acidity as low as 15.5 g/1 can be achieved. The claims specify a solution with a pH of less than 1.5. Nevertheless, it is obvious to one skilled in the art that to minimise the quantity of neutralizing agent required when the ferric iron is precipitated, the acidity of the initial solution must be as low as possible.
In the case where ferric iron is removed from the solution by solvent extraction, it is clear from the results in the paper by Van der Zeeuw, A. J., entitled "Purification of Zinc Calcine Leach Solutions by Exchange Extraction with the Zinc Salt of `Versatic` Acid", published in Hydrometallurgy, volume 2, 1976/1977, pages 275-284, that the acid present in the solution is extracted at the same time as the ferric iron, and it is intuitively obvious to one skilled in the art that the amount of organic solvent required to treat a given volume of solution containing ferric iron and valuable metal or metals will be reduced if the initial acidity is reduced.
The recent invention of the Low-contaminant Jarosite Process (Australian Pat. No. 506591) teaches a method for reducing the acidity of a solution to within the range of 0.1 to 30 g/1 H.sub.2 SO.sub.4 by cooling before the addition of a neutralizing agent, thereby reducing the risk of jarosite being precipitated during the so-called preneutralization step. However, it is apparent from the data presented in Table 1 of the paper by Pammenter, R. V., and Haigh, C. J., entitled "Improved Metal Recovery by the Low-contaminant Jarosite Process", Extraction Metallurgy '81, published by The Institute of Mining and Metallurgy, London, 1981, that for the best operation of the process the acidity of the solution should be as low as possible. The same paper also states that for the best operation of preneutralization, the acidity should be between 3 and 5 grams of H.sub.2 SO.sub.4 per liter (see page 4 of the paper).
In those processes where no neutralizing agent is required in the ferric iron removal step, such as the Low-contaminant Jarosite Process, or where ferric iron is precipitated from solution at high temperature and pressure, or where the iron is removed from an acidic solution by solvent extraction, a reduction in the acidity of the solution entering the iron removal step is advantageous to the process, as indicated by reference to the prior art.
There is thus the need for a new procedure which achieves a lower acidity in the solution containing ferric iron and valuable non-ferrous metals than is currently possible. There is an even greater advantage to be gained if a process could be found which would decrease the amount of acid generated during the ferric iron removal step, as this would decrease the amount of neutralizing agent to be added, where such neutralizing agents are required.